Conductors, for use as coils in generators and motors, are usually insulated by a vacuum-pressure impregnation process. This involves wrapping the conductors with mica tape, and then with a glass binding tape to hold the mica tape on. The wrapped conductor is then placed in a vacuum, with an impregnating resin under pressure. As a last step, the wrapped, impregnated conductor is removed, and heated in an oven to cure the resin.
While this system is used commercially, it requires a great deal of energy for the baking ovens, because the copper in the coils and the iron in the stator frame must be heated along with the resin. Run-off of the resin during cure can occur, and mechanical stresses in the resin can be induced by expansion of the copper during cure and its subsequent contraction during cooling. In many instances, the solvent evaporated during heating contributes to air pollution.
Anaerobic resins are resins which will not cure in the presence of oxygen. Some resins are described as "anaerobic" because they will cure when placed between two oxygen-impervious metal surfaces. These types of resins are commonly used in adhesives. Groff, in U.S. Pat. No. 3,539,438 discloses wrapping a conductor with mica paper and impregnating it with an acrylic resin. Anaerobic type acrylic resins, mixed with peroxy initiators, are disclosed by Toback, in U.S. Pat. No. 3,616,040, for use in adhesive bonding of surfaces, when placed in contact with and between nonporous substrates, generally made of metal. Anaerobic type all acrylic resins have been found to present some problems of shrinkage upon cure, to lack complete thermal stability at high temperatures, and to have only moderately good electrical properties upon cure.
Vogel et al. have developed one component anaerobic-epoxy metal adhesives, based on epoxy resin diacrylates, alkyl or glycol methacrylates, amine accelerators and hydroperoxide initiators, as disclosed in 81 Chemical Abstracts, No. 8, 50530r; 50659q; and 50660h (1974). Izao et al., in 81 Chemical Abstracts, No. 5, 26474e (1974), and Takiyama et al., in 82 Chemical Abstracts, No. 16, 112858k (1975), formulated anaerobic-epoxy adhesive materials, by precooking epoxy resins with acrylic containing reactive species and subsequently blending in acrylic monomer, amine accelerator, and peroxide initiator. Green, in 81 Chemical Abstracts, No. 14, 78994u (1974), synthesized acryloyloxy and methacrylolyloxyhydroxypropylethers as precursors for anaerobicepoxy metal adhesive compositions. These adhesive systems have depended primarily on metal catalyzed cure from the bonding surfaces, rather than cure through oxygen displacement by means of a diffusion gas.
What is needed is a resin system useful for coating and potting electrical conductors, and for coating and vacuum impregnating mica insulated high voltage electrical coils; where the resin system can be cured by a room temperature method, in an oxygen-free gaseous environment, by an oxygen displacement curing mechanism, rather than metal catalysis requiring metal contact with the resin. The resin system should have low shrinkage during cure, good electrical characteristics, i.e., power factor and dielectric constant, good thermal stability, high bond strength with mica, and most importantly, the ability to deep cure without heating.